Diffraction type sound absorber



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DIFFRACTION TYPE SOUND ABSORBER Filed NOV. 30, 1943 6 Sheets-Sheet 1INK/DENT sod/V0 WALL Mei a 4 -I I a 8 Zinnentor Harry 1 01/6070 $1 1424.

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DIFFRACTION TYPE SOUND ABSORBER Filed Nov. 30, 1943 6 Sheets-Sheet 2 0000000 0 o o e 0 0 o o o 9 e 0 00 0 0 0 0 00000 a 0 0 0 o o o 9 0 0 0 0ooo e g m o0 g Q 0 0 0a o o 0 0 a Q 0 0 0 0 0 a o o a d 16 OOOO 0 16 O0000 0 0 09 0 ac 0o 14 oco oo 00 0 0 90000000 6 00-000 ooo oooo 0 0ooooooooo o o oz' l5 ooooooo-oe o oooooooo g o o oo 000006 20 0 2oooooooq 0 20 9 uoobog go 000000000 OOOOQQQQQ 0 3nontor 000060000attorney March 28, 1950 OLSON 2,502,016

DIFFRACTION TYPE souma' ABSORBER Filed Nov. 50, 194:5

6 Sheets-Sheet 3 2*;3 J 19% T fi 2 1 E2 lhwcmor Harry E Olson March 28,1950 H. F. OLSON DIFFRACTION TYPE-scum) ABSORBER 6 Sheets-11991. 4

Filed Nov. :50, 194:5

ISnvcntor Har 1%] 0 L80 n n March 1950 H. F. OLSON DIFFRACTION TYPESOUND ABSORBER 6 Sheets- Sheet 5 Filed Nov. 30, 1943 u (a nventoz $550MMarch 28, 1950 H. OLSON mmuc'rxon TYPE sounn ABSORBER 6 Sheets-Sheet 6Filed NOV. 30, 1943 j m E E N m. 1 ,m R mm A Ma Q A 6 m I t H W A z mfl/ 0W 5 XML. m 5 a H m f w m w m W m 1 mm 1' Kw m m u MM w w 4 Z I 0 iSmemc's FIPMdE/Mff mam-442$ PEI? 555mm @mmmeg Patented Mar. 28, 1950OFFICE DIFFRACTION TYPE SOUND ABSORBER Harry F. Olson, Princeton, N. 1.,assignor to Radio Corporation of America, a corporation of DelawareApplication November 30, 1943, Serial N0. 512,320 13 Claims. (cl.181-33) This invention relates to acoustic absorbers, and moreparticularly to an acoustic absorber of the diffraction type.

The practice of providing sound absorbing materials in rooms,auditoriurns, ofllces, apartments, factories, etc. is well known.Conventional sound absorbing materials used in this connection areusually designed for dual p ses, namely, as a building or wall materialand as a sound absorber. To be satisfactory for wall structures, thesematerials must be quite rigid, and in order to provide suflicientrigidity, the inherent properties of these materials are such as toimpart to them acoustical impedances which are high compared to thecharacteristic acoustic impedance of air in the free atmosphere, beingusually from ten to twenty times as great. As a consequence, a

large amount of incident sound energy is reflected in spots, so tospeak, whereby to take advantage I of the phenomenon known asdiffraction for improving the efliciency of the absorbers. However,existing materials are not particularly suitable for use in this "spofashion, being usually relatively heavy and cumbersome, difllcult toinstall. and ineflicient.

The primary object of my present invention is to provide an improveddifiraction or spot type acoustical absorbing unit which is free fromthe aforementioned and other similar limitations which characterizeprior art absorbers.

More particularly, it is an object of my present invention to provide animproved difl'raction type acoustical absorber which readily lendsitself to use as a spot" type absorber and which can be installed easilyeven by an unskilled person.

Another object of my present invention is to provide an improveddiffraction type acoustical absorber which, for a given amount ofabsorbing material, provides an extremely low reverberation timecompared to absorbers of the prior art.

Still another object of my present invention is to provide an improvedacoustical absorber as above set forth which is highly effective at thelow and high ends of the audio frequency spectrum, as well as in theintermediate range, in contrast to conventional absorbing materials 2which are effective over a rather limited intermediate range only. Thismakes my improved absorbers particularly useful for installations whereeither low or high frequency noises, or both, are in abundance, as infactories, industrial oiilces, plants, restaurants, bowling alleys, andthe like, while being also especially suitable in auditoriums and otherplaces where people congregate in relatively large numbers, ofllces,apartments and the like.

A further object of my present invention is to provide an improvedacoustical absorbing unit as above set forth which, while designedprimarily as an acoustical absorber. may also be made decorative orartistic in appearance for use in homes, theatres, museums, and otherlarge auditoria.

Still a further object of my present invention is to provide an improvedacoustic absorbing unit as aforesaid which can be rendered decorative bythe application of paint to the outer surface thereof without impairingthe absorbing qualities thereof, in contrast to conventional absorbers,the efliciency of which is usually reduced by the application of paintor the like thereto due to the hard surface which the dry paintprovides.

Another object of my present invention is to provide an improvedacoustical absorbing unit of the type set forth which can be easilyremoved for cleaning and the like and equally easily remounted for use.

A further object of my present invention is to provide an improvedacoustic absorber as aforesaid which is extremely light in weight.

It is also an object of my present invention to provide an improvedacoustical absorber which has an extremely high absorption coefficient,which has a much lower cost per unit of absorption than do acousticabsorbers of the prior art, and which is highly efficient in use.

In accordance with my present invention, the absorber comprises a casingwhich encloses a relatively large fluid filled space and the wallstructure of which is constituted at least in part by a material whichis pervious to acoustical waves. This material is of a character suchthat it offers a relatively large dissipative impedance to the passageof sound waves therethrough from the exterior of the casing to theinterior, while the space enclosed by the casing is of suflicientlylarge volume to offer a relatively small impedance to the passage of thesound waves through the casing. The absorber units may be made in theform of box-like structures Or in any other suitable form, and anydesired number of such units may be mounted at suitable points or "spots3 throughout a room which is to be corrected acoustically.

The material of which the casing is made may take various forms. In oneconstruction, this material is constituted by an outer layer ofcardboard having a plurality of openings therein of such dimensions asto permit sound waves to pass therethrough substantially freely, theabsorbing material backingup and lining the cardboard layer. Inthisparticular modification, the absorbing material is constituted by aplurality of layers of thin paper having fine slits therein disposed atrandom and in sufficient communication with each other to provide aplurality of continuous passages which establish communication betweenthe exterior of the casing and the space therein. The inner, sounddissipating layer is of a limp material and may be stapled or otherwisesuitably secured to the outer, rigid, car board layer to provide arigid, self-contained, selfsupporting structure.

In another form of the. invention, the material is constituted by afelted, fibrous material the fibers of which are intertwined insuiliciently compact relation to provide a plurality of passages ofsmall cross-sectional dimensions affording communication between theexterior of the casing and the space therein. In any case, the smallpassages in the absorbing material provide a very high acousticalresistance which dissipates the sound energy passing therethrough, thelarge volume of fluid in the space within the casing having sufficientcapacitance to provide a low impedance to the incoming sound waveswhereby the waves will enter the passages and the energy thereof will bereadily dissipated.

The novel features that I consider characteristic of my invention areset forth with particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation,together with additional objects and advantages thereof, will best beunderstood from the following description of several embodimentsthereof, when read in connection with the accompanying drawings in whichFiguresl and 2 are bar graphs showing, in a general way,-the absorptiveand reflective properties of certain types of sound absorbing wallstructures,

Figures 3, 4 and 5 are similar graphs showing the absorptive propertiesof absorbers formed in accordance with my present invention,

Figure 6 is a central, sectional view through one form of absorbing unitconstructed in accordance with my present invention,

Figure '7 is an enlarged fragmentary view, partly in section, of aportion of the wall structure of the unit shown in Fig. 6,

Figure 8 is a perspective view of a blank from which an absorber such asshown in Fig. 6 may be constructed,

Figure 9 is a view thereof in partially assembled form,

Figures 10 and 11 are perspective views of two completed absorbing unitseach of somewhat different shape and each formed from a blank similar tothat shown in Fig. 8,

Figure 12 is an electric wiring diagram showing, by way of illustration,the electrical system analogous to the acoustical system of Fig. 6,

Figures 1341 and 13b are similar electrical wiring diagramscorresponding to the acoustical system' of P18. 1,

Figures 14a and 14!: are similar wiring dia- 4 grams corresponding tothe acoustical system of each of the units shown in Fig. 5,

Figure 15 is a view showing a plurality of absorbing units in accordancewith my present invention mounted in a room requiring acousticaltreatment,

Figure 16 is a graph showing the reverberation time of the room of Fig.15 both before treatment and after treatment thereof, .1

Figure 17 is a graph showing the comparative absorption coefficients ofseveral prior art acoustic absorbing materials and also of my improvedabsorber, and

Figures 18 to 23, inclusive, are fragmentary sectional views showingother forms of absorber casing wall structures constructed in accordancewith my present invention.

Referring more particularly to the drawings, wherein similar referencecharacters designate corresponding parts throughout, there is shown, inFig. 1, a wall faced or lined with conventional absorbing material I.

The width of the several bars 8, 5 and I is shown in approximatelyproportionate relation to indicate that there is less sound reflected bythe surface of the material I and still less from within the material Ithan is received by this material. A reduction in the amount ofreflected sound by the relatively high impedance absorbing material I ofthe prior art may be obtained by spacing the material I from the wall,as shown in Fig. 2. In practice, however, it is not feasible to providea large space between the absorbing material I and the wall, so that thecondition represented by Fig. 1 is the one usually encountered.

Fig. 3 illustrates diagrammatically the sound absorptive qualities of anabsorber constructed in accordance with my present invention andcomprising a pair of wall members I, 9 spaced from each other a distanceto provide a space ll of relatively large volume. As shown in Fig. 3,the incident sound represented by the bars 3 strikes the front surfaceof the wall I and only arelatively small amount thereof, as shown by thebars H, is transmitted to the space Ill. S uch sound as is transmittedby the wall It then'strikescharacteristic impedance of the air. 3 Itwill be noted that the sound transmitted by;

a single layer of the material of which my improved absorbers are made,as shown by the bars H of Fig. 3, is somewhat higher than thattransmitted by a single layer of the prior art absorbing materials.However, if two layers are used, as shown in Fig. 3, the net amount ofsound transmitted is negligible, as shown by the thin bars II. Byplacing a single sheet 8 of my improved absorbing material at asubstantial distance from the wall to provide a space in of relativelylarge volume, as in Fig. 4, the same effect is produced as in Fig. 3because the sound which is transmitted by the layer of material Istrikes the wall, is reflected thereby back to the material 8, and mustpass through the latter once more. Thus,

the sound passes through the absorbing material l twice before enteringthe room again and therefore almost entirely dissipated.

In this figure, the bars I- indlcate the incident sound and the barsBand 1; indicate the sound reflected by the material I.

When used as a wall material as shown in Fig. 4, the absorption per unitarea of the absorbing material 8 is approximately unity. n the otherhand, if the material is applied in spots, or as discrete units l4spaced from each other as shown in Fig. 5, the efliciency isconsiderably improved due to diffraction of thesound. The absorption perunit area under these conditions is considerably greater than unity.Obviously, therefore, from an economic standpoint, it is desirable totake advantage of the phenomenon of diffraction to improve theefliciency.

One of the ways in which use may be made of diffraction is to scatterthe sound absorbing material throughout the room in the form of small,box-like units. One such unit, which I have termed an acoustic sinkbecause of its ability to absorb the sound energy received by it, isshown in Figs. 6 to 11, inclusive. This comprises a closed, hollowcasing having a composite wall structure which includes an outer layerof cardboard or other suitable sheet material l5 and an inner layer II.Th layer I5 is approximately 3 thick and is provided with a plurality ofopenings l6 which are large enough to permit the passage of acousticalwaves therethrough substantially freely, the openings it being suitablydistributed over the entire area of the layer IS. The inner layer H,which constitutes the absorbing material proper, is suitably secured tothe inner surface of the outer wall material [5 and may be constitutedby any one or more of a variety of sound absorbing materials which Ihave found suitable. Among these is one constituted by approximately 60layers of very thin paper having a plurality of natural holes orperforations therein which are arranged at random, so that they do notline up, but which have suflicient communication with each other toprovide a plurality of continuous, irregular passageways through whichsound waves may pass from the exterior of the unit It to the interiorthereof, as clearly shown in Fig. 7. Thus, the material I! forms asystem of long, narrow slits through which the sound waves must pass toreach the space within the unit I4.

The sound absorbing layer I1 is itself a limp material and therefore notself-sustaining. It is for this reason that I provide the outer, rigidlayer l5 of cardboard or the like and secure the absorbing layer llthereto by means of staples ill or in any other suitable manner, as bystitching, or the like. After securing the sound absorbing layer II tothe outer, supporting layer 15 to form the wall structure of theabsorbing unit I, the composite wall structure may be blanked out in theform shown in Fig. 8 and thereafter assembled by bending up the sides,top and ends, as shown in Fig. 9, to provide a closed casing, as shownin Figs. and 11, the latter form being cubical, and the former being inthe shape of a rectangular parallelepiped.

The casing it thus provided by the composite wall structure I5, I!encloses a cavity or space IQ of relatively large volume which is filledwith air or any other suitable, compressible fluid. If the inertance ofthe air in the openings I6 is M1, the acoustic resistance ofiered by theopenings I6 is n, the inertance of the passageways formed by the slitsor the like in the absorbing layer I1 is Mz. the acoustical resistanceoffered thereby is re, and the acoustical capacitance of the air in thespace I9 is C, then the corresponding elec' trical system will be asshown in Fig. 12. The acoustic elements which control the performanceinertance M1 and the resistance r; are small compared to the inertanceM: and the resistance n. The impedance of the absorber or sink I4 is thesum of the input acoustic resistance and the acoustic reactancedesignated, respectively, as r. and Is- The eiTect of difl'raction inimproving the emciency of acoustic absorption by the use of an absorbingunit or sink as above described will be readily understood by means ofthe following theoretical consideration of the acoustic impedanceconcept when considered in connection with Figs. 1, 3, 13a, 13b, 14a and1411: In the case of Fig. 1, it is assumed that the surface of thematerial l is relatively large, (that is, that it covers the walls orthe ceiling or both in a room). Under this condition, the acousticresistance TA of the air is its characteristic impedance. The acousticimpedance of air is 42 acoustical ohms per square centimeter. Theacoustic resistance and acoustical reactance of the absorbing material Iare designated as m and :m, respectively, and the correspondingelectrical system is shown in Fig. 13a. The dissipation or power P whichis lost in a sound wave of pressure 1: due to TM is given by theequation:

u? A+ M)+ M' (1) Referring to Equation 1, it will be seen that maximumabsorption is obtained when am=0 (corresponding to Fig. 13b) and TA=TM.For these conditions, Equation 1 reduces to Reference will now be madeto the improved acoustic sink ll of my present invention andparticularly to Fig. 5. The air impedance contains both a resistive andreactive component. Therefore, in this case, the sound waves areconvergent. The resistive component of the air is designated as TA andthe reactive component of the air due to its inertance is designated MA.The

acoustic circuit is shown in Fig. 5 and the corresponding electricalanalogue in Fig. 14a. The impedance of the acoustic sink I4 is composedof the acoustic resistance r, and acoustic reactance as which may beobtained from Fig. 6. In order to obtain a direct comparison withconventional acoustic materials, it may be assumed that the reactivecomponent $5 is negative .and equal in magnitude to the acousticreactance due to MA. The acoustic circuit then is reduced to onecorresponding to Fig. 14b.

As a specific example, let it be assumed that the sink is approximatelya cubic foot in volume and that the frequency under consideration iscycles per second. Under these conditions, the acoustic resistance TA isapproximately 4 acoustic ohms. To obtain maximum absorption, Ts shouldalso be equal to 4 acoustic ohms. For this condition, the dissipation inTs is The preceding analysis shows that the characteristic impedance ofthe air is reduced by using unit sound absorbers and separating them inspace. The characteristic impedance of the air may be considered to bethe internal impedance of a generator which has an internal drivingpressure 12. As the above analysis shows, this means that it is possibleto absorb more sound per unit area under these conditions because theeffective internal impedance of the generator is smaller. Thedesirability of building the absorbers in units and appropriatelydistributing them throughout the room is therefore apparent. In thisway, maximum use can be made of the absorbing material.

As pointed out heretofore, most absorbing materials known heretofore aremade to serve dual purposes. They are designed to serve both as wallmaterials and as acoustic absorbing materials. For this reason, acousticmaterials of the prior art are not as efficient acoustically as it ispossible to obtain by designing for maximum sound absorption.Furthermore, it is rather difflcult to incorporate a large space betweenthe wall and the sound absorbing material, and therefore absorbingmaterials are usually placed directly against or very close to the wall,as in Fig. 1. Consequently, the acoustic reactive component :m of mostmaterials is usually quite large, and this results in a further loss inefliciency.

Referring again to Fig. 6, it will be seen that, in the acoustic sink ofmy present invention, the reactive components are due to the inertanceof the holes IS, the inertance of the material I! and the acousticcapacitance of the volume of air in the space l9. In general, thereactance due to the inertance is quite small. Therefore, thecontrolling reactance is due to the acoustic capacitance. The acousticcapacitance of a volume of air is given by the equation where p=densityof air c=velocity of sound V=volume of the enclosure (the space is inthe present case).

Equation 4 shows that the acoustic impedance can be made small by makingthe volume of the space H] large. This volume can be made practicallyany appropriate value in the acoustic sink. The most logical value wouldbe such that the reactance due to the acoustic capacitance would besubstantially equal to the reactance due to the combined inertance ofthe holes IS, the material I'! and the air load at the lowest frequencyto be absorbed.

Referring to the acoustic circuit of Fig. 6, the acoustic resistance Tldue to the holes I6 is negligible. Therefore, the important acousticresistance is re. The acoustic resistance due to the material I! shouldbe equal to the average characteristic acoustic resistance of the air inorder to obtain maximum absorption. The acoustic resistance of thematerial depends upon the type of material used. A slit can be made toexhibit practically a pure acoustic resistance as shown by the followingequation:

p=density, in grams per cubic centimeter,

d=thickness of the slit normal to the direction of flow, in centimeters,

l=-width of the slit normal to the direction of flow,

in centi eters,

w=length of the slit in the direction of flow, in

centimeters,

(0:21), and

=frequency, in cycles per second.

This is the type of resistance shown in Figs. 6 to 11, inclusive. Theair enters the holes IS in the outer, cardboard wall portion l5 andfollows a tortuous path through the layers of paper or the like .41, asclearly shown in Fig. 7. The holes in the paper allow the air to passfrom one layer of paper to another layer, these layers being shown bysingle broken lines in Fig. 7 for the sake of clearness. The slits areformed by the layers of paper.

An acoustic resistance can also be obtained from an aperture or tube ofsmall diameter. The

acoustic impedance of a tube of small diameter is given by whereR=radius of the tube, in centimeters, =viscosity coefficient (1.86x10-for air), w=21rf,

f=frequency, in cycles per second, l=length of the tube, in centimeters,and =density, in grams per cubic centimeter.

In the case of cloth or other finely screened material, the material hasholes of small diameter between the crossed threads. Equation 6 showsthat such a material constitutes an acoustic resistance due to thesesmall holes. The magnitude of the acoustic resistance may be adjusted byan appropriate size of holes in the apertures between threads and by thenumber of layers of cloth or the like. It has been found that cloth is avery good acoustic resistance, and a sink employing a number of layersof cloth or similar material l'la as the sound absorbing layer is shownin Fig. 18.

Hair felt or a material known commercially as Ozite" is a combination ofslit and hole type of acoustic resistance. A wall structure employinghair felt or the like Nb and suitable for use as the casing of myimproved absorbing unit II is shown in Fig. 19. This is also a goodacoustic resistance, the effect being enhanced by vibration of the hairsor fibers themselves.

Porous material, that is, material with fine holes, may also be madefrom a porous paper such as blotting or filter paper, as shown in Figs.20, 21, 22 and 23. The material may or may not be sufficiently rigid tobe self-sustaining, and if it is not, it may be secured to an outer,cardboard layer as above described with reference to Figs. 6 to 11,inclusive. One material which I have found especially suitable is afelted material made from wood pulp fibers and sulphite, the fibersbeing accreted by suction from a pulp solution in known manner. Thismaterial, like blotting paper and the like, is constituted by aplurality of intertwined fibres of various lengths and arrangedindiscriminately in random directions to provide a plurality ofirregular, random passages of very small cross-sectional dimensions.These passages afford communication between the exterior of the casingcomprising each of the units I4 and the space is enclosed thereby andprovide the requisite dissipative impedance to the acoustical energyreceived thereby.

In Fig. 20, the wall structure of the casing is constituted by a porousmaterial llc of the type just previously described, the material l'lcbeing preferably corrugated and glued to the outer cardboard layer l5.If desired, an inner cardboard layer may be also glued to the corrugatedmaterial llc, as shown in Fig. 21. The cardboard layer lie is similar tothe outer cardboard layer l5, being provided with openings Ilia whichcorrespond to the openings I6.

In the modification of Fig. 22, the sound absorbing material "11 isprovided with ridges which may be glued to the outer cardboard wallmember Is. In general, the felted type of sound absorbing material hassuflicient rigidity to be self-sustaining, and does not require thesupporting cardboard layer I 5. In such case, the cardboard layer may beentirely dispensed with and the acoustic sinks made solely of a singlelayer of porous material We, as shown in Fig. 23.

Fig. 17 shows the relative absorption coefilclents of three conventionalsound absorbing materials and also my improved acoustic sink. The threecurves B. C and D show the absorption coeflicients of three standardmaterials which may be identified, respectively, as AMA type V, AMA typeVI, and AMA type IV. It will be seen that these materials are quiteineffective in the lower region of the audio spectrum and are onlyfairly effective in the region between 500 C. P. S. and 1000 C. P. S.,falling off again above 1000 C. P. S. The curve A, on the other hand,clearly shows that my improved acoustic sinks not only possess a higherabsorption coeflicient in the mid-frequency range, but that they arevastly more effective in both the lower and the higher frequency ends ofthe audio spectrum.

In Fig. 15, there is shown a room treated with a plurality of acousticsinks M in accordance with my present invention. The sinks are allmounted adjacent to the ceiling, certain ones being mounted on the wallsof the room in spaced relation thereto by means of brackets or the like25, and certain other ones being mounted on the beams or panels 26 whichdepend from the ceiling. The sinks mounted on the walls are spacedtherefrom to aiford the incident sound ample opportunity to reach allthe surfaces of the sink so that advantage may be taken of diffraction.The curve E of Fig. 16 shows the reverberation time of the room depictedin Fig. 15 before treatment with the sinks of my present invention. andthe curve F shows the reverberation time of the same room aftertwenty-four such sinks were mounted therein at distributed points orspots in the room. It will be noted from the curves of Fig. 16 that thereverberation time was reduced in the low frequency region fromapproximately 3 seconds to approximately 0.6 second, and that averysubstantial reduction was also obtained in the reverberation time at thehigher frequencies.

From the foregoing description, it will undoubtedly be apparent to thoseskilled in the art that I have provided an improved sound absorbing unitwhich is not only highly efficient in use, but which is very compact andis easy to install and to take down. Although I have shown and describeda number of modifications of my present invention, it should be apparentto those skilled in the art that many other variations thereof, as wellas changes in the ones described, are possible. For example, where thematerial 11 is of a rigid nature, it may be made up into a box-likestructure separately from the outer,

10 cardboard layer It and placed within the box formed by the latter,after which the entire unit is sealed or otherwise suitably securedalong the edges to provide a unitary, self-contained structure. Wherethe sound absorbing material is of the limp type, it may be desirable,in some cases, to provide an inner, as well as an outer supporting wallstructure, as in Fig. 21. In such case, an inner boa; which is nextadjacent to the space l9 may first be formed and placed within the outerbox formed by the layer 15, the material l'l being interposed in thespacev therebetween. So far as the layers !5 and 15a are concerned. the

openings l6 and 18a therein may be circular, F as shown, or they may beof any other suitable shape. In all forms of the invention thus fardescribed, a minimum amount of material 11, Ha, etc. may be used toprovide the requisite acoustic resistance because it is terminated bythe acoustic capacitance of the space l9. In other forms of my presentinvention, a single layer of relatively rigid, non-porous material maybe used for the casing wall structure and a plurality of diaphragms orother similar vibratory members mounted over openings therein, thediarhragms or the like being exposed at the front to the atmosphere andat the rear to the space 19. When incident sound strikes suchdiaphragms, the latter are set into vibration and offer suflicientimpedance to the sound to dissipate the energy thereof. Thus, in asense, the vibratory diaphragms render the wall structure pervious tothe sound waves in a manner similar to the porous structures.

Other changes and modifications, including pyramid-like, conical,spherical and other similarly shaped hollow casing structures, will, nodoubt, readily Suggest themselves to those skilled in the art. Itherefore desire that my invention shall not be limited except insofaras is made necessary by the prior art and by the spirit of the appendedclaims.

I claim as my invention:

1. A diffraction type acoustic absorber comprising a self-contained,closed casing enclosing an air-filled space therein, said casing beingconstituted by an acoustically porous material having a plurality offine passages therethrough of such fine dimensions that they offer arelatively large dissipative impedance to the passage of said wavestherethrough between the exterior of said casing and said space, andsaid space having a volume of such magnitude that the air therein offersa relatively small impedance to the passage of said waves through saidcasing.

2. A diffraction type absorber according to claim 1 characterized inthat said casing comprises a laminated wall structure of which saidmaterial forms a part.

3. A diffraction type absorber according to claim 1 characterized inthat said casing has a wall structure comprising a layer having aplurality of openings therein of sufficient size to permit the passageof sound waves therethrough substantially freely and a layer of saidmaterial lining said first named layer.

4. A diffraction type absorber according to claim 1 characterized inthat said casing has a wall structure comprising a layer having aplurality of openings therein of sufficient size to permit the passageof sound waves therethrough substantially freely and a layer of saidmaterial lining said first named layer, and characterized further inthat said material is disposed between said first named layer and saidspace.

' 531d space.

6. A diffraction type absorber according to claim 1 characterized inthat said passages are of slit-like formation and are arranged at randomin said material.

7. A ditlraction type absorber according to claim 1 characterized inthat said material is constituted by a plurality of layers of thinsheets each provided with a plurality of slit-like openings therein,said openings being arranged at random throughout said sheets but havingsufficient communication with each other to provide a plurality ofcontinuous passageways between the exterior of said casing and saidspace therein constituting said passages.

8. A. diffraction type absorber according to claim 1 characterized inthat said material is constituted by a plurality of intertwined fibersarranged indiscriminately in random directions and in relatively compactrelationship whereby to provide said plurality oi passages each of smallcross-sectional dimension.

9. An acoustic absorber according to claim 1 characterized in that saidpassageways are disposed at random throughout said wall structure.

10. An acoustic absorber according to claim 1 characterized in that saidpassageways are irregular in shape and extend in random directionsthrough said wall structure.

11. An acoustic absorber according to claim 1 characterized in that saidbody has an overall acoustic impedance approximately equal to thecharacteristic acoustic impedance of air in free atmosphere whereby aminimum oi. the sound wave energy received thereby is reflectedtherefrom.

12. An acoustic absorber according to claim 1 characterized in that thereactance ofiered by the asoaow air in said space due to the capacitancethereof is substantially equal to the reactance provided by said wallstructure and the load thereon at substantially the lowest frequency tobe absorbed by said body.

13. A diflraction type acoustic absorber comprising a self-contained,closed casing enclosing a fiuid filled cavity therein, said casingcomprising a unitary structure capable of being suspended in the ambientfor reception of acoustical wave energy incident thereon from anydirection and being constituted by an acoustically porous materialhaving a plurality oi fine passages therethrough of such fine dimensionsthat they ofler a relatively large dissipative impedance to the passageof said waves therethrough between the ambient and said cavity, and saidcavity having a volume or such magnitude that the fluid therein offers arelatively small impedance to the passage of said waves through saidcasing.

HARRY F. OLSON.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 1,385,741 DillonJuly 26, 1921 1,483,365 Mazer Feb. 12, 1924 1,7 26,500 Norris Aug. 27,1929 1,816,769 Fisk July 28, 1931 1,832,741 Sersen et al Nov. 1'7, 19311,851,208 Nicolson Mar. 29, 1932 1,903,201 Steur et al Mar. 28, 19331,926,679 Kellogg et al Sept. 12, 1933 2,101,568 Woodbury Dec. 7, 19372,160,638 Bedell et al May 30, 1939 2,221,001 Lucius Nov. 12, 19402,308,869 Eckardt Jan. 19, 1943 FOREIGN PATENTS Number Country Date8529/27 Australia Aug. 7, 1928 102,914 Australia Jan. 30, 1938 406,384Great Britain Mar. 1, 1934 409,502 Great Britain May 3, 1934 Italy uJuly 17, 19 0 Certificate of Correction Patent No. 2,502,016 March 28,1950 HARRY F. OLSON It is hereby certified that error appears in theprinted specification of the above numbered patent requiring correctionas follows:

Column 7, lines 41 and 42, for the equation O= read 0= pc 7 p6 and thatthe said Letters Patents should he read with this correction thereinthat the same may conform to the record of the case in the PatentOffice.

Signed andsealed this 12th day of September, A. D. 1950.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.

